The BBC has defended the latest issue of Focus, its science and technology magazine, after the editor devoted a dozen pages to a book written by himself.

The April issue carries extensive coverage of the forthcoming book, The Science of Doctor Who, written by Paul Parsons.

References to Parsons' book, which explains how aspects of the long-running sci-fi drama such as the Tardis or the Cybermen relate to real science, or its title, feature on the front cover, the contents (P3), the editor's letter (P5), an eight-page article (starting P32), as a competition prize (P39), a subscription offer (P40) and a book review (P70).

But the book, which is released next month, is branded an "unofficial guide" because publisher Icon Books is not part of the BBC.

"This month we've turned Focus's scientifically critical eye on the new series of Doctor Who, due to air imminently on BBC One," Mr Parsons writes in his editorial.

"The success of books such as Lawrence Krauss's bestseller The Physics of Star Trek demonstrate that lots of SF fans enjoy nothing better than discovering how their much loved fictional universe might or might not work.

"Krauss's book, for example, sold 250,000 copies on America alone.

"For the super keen, my book The Science of Doctor Who is published on April 6 and you can get a free copy by subscribing to Focus - details on page 40."

BBC Origin, a division of BBC magazines based in Bristol, publishes the magazine.

A BBC spokeswoman defended the magazine's coverage, saying the book featured so frequently for "viable commercial and editorial reasons".

"As a BBC-branded magazine the government requires that Focus supports BBC Television programming," the spokeswoman said.

"With the new TV series of Doctor Who to be broadcast in April, the April issue of Focus features The Science of Doctor Who - a feature with an angle suitable to the magazine's popular science and technology content. Given that Paul is a Doctor Who expert it was only natural that he write this article.

"BBC Worldwide is the commercial arm of the BBC. As such, all profits are returned to the BBC for the benefit of the licence fee payer."

Origin Publishing bought Focus from National Magazines in 2001. BBC Worldwide, the commercial arm of the BBC, bought Origin Publishing in 2004 and rebranded Focus as BBC Focus last year.

The magazine increased its year-on-year circulation in the last six months of 2005 by 8% to 57,306.

At the time, Andy Benham, the publishing director of BBC Origin's magazine division, said: "Focus has gone from strength to strength and has recorded yet another increase this ABC.

"I'm particularly happy that the growth has come from both the UK newsstand and subs, which shows the build-up of trust in the title since the redesign in early 2005."